The invention relates generally to chiller refrigeration systems and, more particularly, to separation of lubricant from refrigerant in a compressor of a chiller refrigeration system.
Refrigerant systems are utilized in many applications to condition an environment. The cooling or heating load of the environment may vary with ambient conditions, occupancy level, other changes in sensible and latent load demands, and as the temperature and/or humidity set points are adjusted by an occupant of the environment.
Use of a variable speed drive for the compressor motor improves the efficiency of refrigerant systems. Often, the compressor need not be operated at full speed, such as when the cooling load on the refrigerant system is relatively low. Under such circumstances, it might be desirable to reduce the compressor speed, and thus reduce the overall energy consumption of the refrigerant system. Implementation of a variable speed drive is one of the most efficient techniques to enhance system performance and to reduce life-cycle cost of the equipment over a wide spectrum of operating environments and potential applications, especially at part-load conditions.
However, compelling reliability concerns limit the allowable compressor speed reduction. In particular, inadequate lubrication of the compressor elements such as bearings may present a problem at low operating speeds. Speed dependent reliability concerns arise because damaging contact may occur between two surfaces in close proximity depending on their relative speed and the viscosity of the lubricant between them. As the speed is reduced, the viscosity of the lubricant must be increased to maintain a separating film between the two surfaces. Lubricant viscosity levels that occur in conventional compressor lubrication systems, which are designed for operation at relatively high constant speeds, are not sufficient to ensure reliability at the lowest speeds desired for variable speed operation.
Most oils used in refrigerant screw compressors form a solution of refrigerant and oil. Refrigerant dilutes the oil, lowering the viscosity of the resultant oil-refrigerant solution compared to the viscosity of pure oil. The amount of refrigerant dissolved in oil in a stable solution is a chemically determined function of pressure and temperature. Suitable changes in pressure and temperature of the oil-refrigerant solution, usually pressure reduction and temperature increase, can cause refrigerant to out-gas from the solution as a new equilibrium state develops. Such occurrences of out-gassing generally increase viscosity because they reduce the level of dilution. Complete out-gassing required to reach a new equilibrium state is not instantaneous. Time required can be reduced somewhat by agitating the lubricant during the out-gassing process.
A known method of increasing viscosity of refrigerant-diluted lubricants that is currently used in some conventional compressors and in variable speed compressors with limited speed range introduces pressure reduction in the lubricant flow prior to its introduction to bearings. This is typically accomplished by venting the housing cavity containing the bearings to a relatively low pressure region within the compressor and by locating an orifice in the lubricant flow path upstream of bearings. The flow restriction imposed by the orifice introduces a pressure drop that may induce some out-gassing of refrigerant. While this approach offers some increase in lubricant viscosity, it has been found to be insufficient to allow operation to the lowest speeds desired.
Due to the minimum speed limitation that must be imposed to ensure reliability, some of the energy efficiency that could be potentially provided by the variable speed drive is essentially eliminated. Thus, there is a need to provide a compressor that can reliably operate at a lower speed than what can be achieved with current designs.